Unlike lesions that affect only one DNA strand, which can simply be excised, inter-strand crosslinks (ICLs) involve both strands of DNA, blocking the essential processes that require translocation along the DNA, namely DNA replication and transcription. In addition to this physical constraint on DNA, ICLs require repair of damage on both strands of the DNA. Comparative studies ranking in vitro and in vivo genotoxicity of large sets of compounds have ranked crosslinking agents among the most toxic (Lohman, 1999).
In actively dividing cells, the replicative machinery will encounter an ICL during S-phase, which will act as a physical barrier to replisome progression. This is thought to be a prevalent mechanism for sensing ICL lesions. In this setting, ICL repair involves the collision of a replication fork with the lesion as a trigger to initiate repair (Haynes et al., 2015; Raschle et al., 2008).
It has been assumed that this replication-dependent mode of repair should be sufficient to cope with and repair ICLs in a timely fashion. However, in non-proliferating cells such as post-mitotic differentiated cells or quiescent stem cells, endogenously generated ICLs need to be repaired in the absence of DNA replication. In this situation, if the ICL is positioned in an actively transcribed gene, a similar sensing process could take place following stalling of the RNA polymerase, with transcriptional blockage acting as the initiating event. Finally, it is thought that distorting ICLs can be recognized in the absence of collision with the replication or transcription machinery moving along DNA (Vogel et al., 1996).
Because ICLs are profoundly cytotoxic, especially for replicating cells, ICL-inducing agents are widely used for cancer treatment, in particular mitomycin C, nitrogen mustard, and platinum compounds (Deans and West, 2011). These are front-line chemotherapeutic agents used to treat a wide range of solid tumours and blood cancers. ICL-based chemotherapy is effective in treating leukaemia or breast and ovarian cancers, and especially testicular cancer with a high cure rate. However, the effectiveness is limited by side effects such as renal toxicity, neuropathy, and development of further tumours. Additionally, tumours often develop resistance to ICL-inducing agents. This chemoresistance is partially due to the restoration of the ICL repair mechanism in which the cell was initially deficient. Overcoming these limitations would greatly enhance the effectiveness of treatment, but the underlying mechanisms are unclear.
Improving our understanding of the mechanisms of ICL repair, and the implications for both cancer development and treatment is extremely important both for patients with ICL repair-related diseases and also for those undergoing ICL-based chemotherapeutics. A better knowledge of the ICL repair mechanism will allows us to move towards rational therapeutic targeting the ICL repair-response in tumors. Our studies, carried out in both human cells and mouse model, constitute a complementary approach for the analysis and understanding of the various mechanism of ICL repair in higher eukaryotes. Particular attention is placed on the development of new in vivo assays to dissect the molecular mechanisms underlying the repair of ICLs. Results from our investigations in cells and mouse provide a solid base to extend our analyses to patient tumours in close collaboration with the Institut Paoli-Calmettes.
Deans, A.J., and West, S.C. (2011). DNA interstrand crosslink repair and cancer. Nature reviews. Cancer 11, 467-480.
Haynes, B., Saadat, N., Myung, B., and Shekhar, M.P. (2015). Crosstalk between translesion synthesis, Fanconi anemia network, and homologous recombination repair pathways in interstrand DNA crosslink repair and development of chemoresistance. Mutation research. Reviews in mutation research 763, 258-266.
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Raschle, M., Knipscheer, P., Enoiu, M., Angelov, T., Sun, J., Griffith, J.D., Ellenberger, T.E., Scharer, O.D., and Walter, J.C. (2008). Mechanism of replication-coupled DNA interstrand crosslink repair. Cell 134, 969-980.
Vogel, E.W., Nivard, M.J., Ballering, L.A., Bartsch, H., Barbin, A., Nair, J., Comendador, M.A., Sierra, L.M., Aguirrezabalaga, I., Tosal, L., et al. (1996). DNA damage and repair in mutagenesis and carcinogenesis: implications of structure-activity relationships for cross-species extrapolation. Mutation research 353, 177-218.